1. Field of the Invention
The present invention relates to a lens-fitted photo film unit with a stop changing device for changing the aperture size of an aperture-stop, and more particularly to a lens-fitted photo film unit that can provide good quality pictures in any photographic conditions.
2. Background Arts
A variety of lens-fitted photo film units have been produced and sold in the market. The lens-fitted photo film unit, hereinafter called the film unit, is preloaded with a roll of unexposed photo filmstrip, and is provided with simple photographic mechanisms. For the purpose of making the film unit as handy and economy as possible, the film unit conventionally uses a simple taking lens whose focal length, focal point and aperture size are fixed. In order to obtain moderate focusing conditions in a wide subject distance range from about 1 m to the infinity, the taking lens is designed to have a large depth of the field, and the f-stop number or f-number is set at around xe2x80x9cf/10xe2x80x9d. With such a fixed f-number, photographs with adequate exposure conditions can be obtained only in limited photographic conditions. To solve this problem, most of the film units is provided with a flash device. In correspondence with the f-number of around xe2x80x9cf/10xe2x80x9d, the guide number of the flash device is set at around xe2x80x9c10xe2x80x9d. That is, the intensity of the flash light is relatively high.
With such a flash light, however, the difference in brightness between a main subject and a background becomes very large in those scenes where the flash light merely reaches the main subject and does not reach the background, and the brightness of the background is relatively low. In that case, if the photographed picture is printed in a condition adjusted to the exposure condition of the main subject, the background would become too dark.
To obtain satisfactory exposure conditions in the flash photography with respect to the background and the main subject as well, Japanese Laid-open Patent Application Nos. 8-114836 and 10-333287 disclose a film unit wherein the aperture size of the aperture-stop is enlarged when the flash photography is selected, for example, by switching over a stop blade in cooperation with a flash charge switch. Thereby, it becomes possible to avoid overexposure in those scenes where the subject brightness is so high that the flash light is not needed. On the other hand, where the subject brightness is low, a sufficient exposure amount is obtained by using the flash light and the large size aperture in combination.
However, because the decision as to whether the flash light is necessary or not depends on the photographer""s subjectivity, the risk of resulting improperly exposed photographs is relatively high. Moreover, where the main subject exits in a near range of about 1 m to 2 m, the main subject would be photographed in an overexposed condition when the large size aperture is used in the flash photography.
There have been suggested many film units that measure the subject brightness and change the aperture size of the aperture-stop depending upon the subject brightness. However, even in such a film unit, if the combination between the f-number, the shutter speed, the film speed and the guide number of the flash device is inappropriate, proper exposure conditions cannot be obtained.
In view of the foregoing, an object of the present invention is to provide a film unit that prevents overexposure of the main subject in the flash photography if the main subject exists in the near range, and also achieves adequate exposure conditions with respect to the background as well as the main subject in any scenes.
Another object of the present invention is to provide a film unit, of which performance factors, including f-number, shutter speed, film speed and guide number of a flash device, are optimized as a whole.
According to an aspect of the present invention, a lens-fitted photo film unit comprises a flash device for projecting a flash light toward a subject; a flash light control device that measures a light amount reflected from the subject while the flash device is projecting the flash light, and stops the flash device from projecting the flash light when the measured light amount reaches a predetermined value; and a stop changing device that changes over the aperture size of an aperture-stop at least between a large size and a small size.
Since the flash light control device stops the flash device from flashing when the light reflected from the subject reaches the predetermined amount, overexposure of the subject is prevented in the flash photography even if the subject exists in the near range.
According to a preferred embodiment, the stop changing device cooperates with a flash ON-OFF member that is manually operated to turn the flash device on or off, such that the stop changing device sets the aperture-stop at the large size while the flash device is on, or at the small size while the flash device is off.
Because the aperture-stop is set at the large size in the flash photography, the background is prevented from being too dark in comparison with the lightness of the main subject.
According to another aspect of the present invention, a lens-fitted photo film unit comprises a flash device for projecting a flash light toward a subject; a flash light control device that measures a light amount reflected from the subject while the flash device is projecting the flash light, and stops the flash device from projecting the flash light when the measured light amount reaches a predetermined value; and a stop changing device that measures a subject brightness and changes over the aperture size of an aperture-stop at least between a large size and a small size depending upon the measured subject brightness.
Since the aperture-stop is changed over depending upon the subject brightness, the photographer does not need to decide whether the aperture-stop is to be set at the small size or the large size. Even when the flash light is used for a relatively bright subject, overexposure is prevented because the flash device is stopped from flashing when the light amount reflected from the subject reaches the predetermined value.
In order to obtain adequately exposed photographs under any photographic conditions, the present invention suggests determining an f-number of the taking with the large aperture-stop size to satisfy the following condition (1), and an f-number of the taking lens with the small aperture-stop size to satisfy the following condition (2):
6xe2x89xa6AV+TVxe2x88x92SV+5xe2x89xa611.5xe2x80x83xe2x80x83Condition (1)
10.5xe2x89xa6AV+TVxe2x88x92SV+5xe2x89xa615xe2x80x83xe2x80x83Condition (2)
wherein AV=log2(f-number2), TV=log2(1/shutter speed), and SV=log2(0.32xc2x7film speed).
According to a preferred embodiment, the f-number of the taking with the large aperture-stop size is determined to be not more than f/8.0, and the f-number of the taking lens with the small aperture-stop size is determined to be not less than f/14. It is preferable to provide the film unit with a flash device whose guide number is equal to or more than 4 but less than 8.